The present invention relates to optical modules for optical communications, and more particularly, to an optical module used in the transmitting section of an optical transmitter operating at high transmission rates.
The optical transmission modules using a semiconductor laser are one of the key devices in transceivers for optical-fiber transmission. Along with the proliferation of broadband networks in recent years, optical transmission modules have been speeded up and types up to 10 Gbits/s in bit rate are coming to be most commonly used. Optical transmission modules suitable for the above applications are required to be more compact and less expensive as well as to achieve higher transmission waveform quality.
Japanese Patent Laid-open Nos. 2001-257412 and 2001-308130 describe the modules that simultaneously ensure both reduction in small-signal reflection coefficient (S11) with respect to a radiofrequency input characteristic impedance of 50 Ω (ohms) and the retention of a 3-dB band in the small-signal passage characteristics (S21) of the optical modulator of an electroabsorption optical modulator integrated laser diode. The above modules are realized by properly conditioning the relationship in inductance between a first bonding wire for connecting the modulator and signal line of the modulator, and a second boding wire for connecting the modulator and a matching resistor.
Also, the “ASIP 1310 nm EML TOSA”, a pamphlet of ASIP Inc., lists electroabsorption optical modulator integrated laser modules that each contain a CAN-type package(s) with a driving impedance of 50 Ω and a termination resistance of 100 Ω.
To further improve transmission waveform quality of an optical transmission module, it is necessary not only to ensure the retention of a 3-dB band in the small-signal passage characteristics (S21) of the optical modulator, but also to optimize the small-signal passage characteristics (S21) by adjusting the peaking characteristics of S21 to spread transmission waveform mask margins relative to ITU-T provisions.
According to studies by the inventors, for optical module driving with an ideal 50-Ω signal source (e.g., measuring-pulse pattern generator), it has been within a sufficiently possible range to optimize the small-signal passage characteristics (S21) in Japanese Patent Laid-open No. 2001-257412 and spread transmission waveform mask margins, by properly conditioning the relationship in inductance between the first bonding wire and the second boding wire for connecting the modulator and the matching resistor. The optimization, however, prevents the small-signal reflection coefficient (S11) from being sufficiently reduced, thus resulting in an impedance mismatch.
For driving ICs currently available in the market, especially for long-distance transmission applications, a bias voltage signal of nearly 1 V and a driving signal with a large voltage amplitude of 2 Vpp or more are required as the driving signals for an optical modulator. Accordingly, output impedance usually departs from 50 Ω during IC operation. In that case, an impedance mismatch occurs across the 50-Ω transmission line connecting the driving IC and the optical modulator. Therefore, a multipath reflection voltage is superimposed on the driving voltage signals and the output waveforms of the optical modulator become disturbed. As a result, transmission waveform mask margins deteriorate.
One of the methods available to solve the above problem is by building the driving IC into an optical transmission module. In this case, the effects of multipath reflection can be suppressed for better transmission waveforms by minimizing the length of the 50-Ω transmission line connecting the driving IC and the optical modulator. However, since the driving IC requires a large number of control terminals and power supply terminals, another problem arises from building the driving IC into an optical transmission module. That is to say, compared with the number of terminals under a non-built-in condition of the driving IC, the number of terminals in the optical transmission module with the driving IC built therein increases, which makes dimensional reduction of the module difficult.